MARC trains, operated by the Maryland Transit Administration, will make nine Saturday round trips and six Sunday round trips between Penn Station in Baltimore and Union Station in Washington, beginning Dec. 7.

The weekend trains will make BWI more accessible to customers than ever, Wiedefeld said.

An estimated 3 percent of the passengers who start or end their travel at BWI arrive or leave the airport via MARC or Amtrak, said Jonathan Dean, an airport spokesman. About 70 percent of BWI's passengers start or end their travel there as opposed to connecting to other flights at the airport.

Richard Aboulafia, an aviation analyst with the Teal Group Corp. in Fairfax, Va., noted that there are other weekend mass transit options for getting to BWI from the Washington area already, including Amtrak and bus connections from Washington's Metro system.

Still, MARC will be another cheap access route for an airport that caters to non-business travelers with less to spend, he said.

"It lowers the price point, increasing accessibility, and that's good for an airport that relies on leisure passengers," he said.

Last year, BWI passed Dulles International Airport in passenger count, becoming the nation's 22nd-busiest airport, according to Airports Council International. Passenger volume at BWI rose 1.25 percent to nearly 22.7 million in 2012, compared with Dulles' 22.4 million, a decline of 2.89 percent.

The airport does not track traffic by day, so statistics are not available for the percentage of fliers at BWI who arrive or depart on weekends, Dean said.

But he said airport officials "have heard from many customers over the years that they would use MARC if weekend service was offered, so we anticipate that the number of BWI passengers using MARC rail will increase with the addition of weekend service."

Wiedefeld said he hopes the increased access will help all of the airports in Baltimore and Washington attract more fliers, not just win customers away from one another.

"We're the third-largest region in the country in terms of airport travel, so it's good if we're all rising," he said. "It doesn't help if we take passengers from another airport or another airport takes passengers from us."

Kimberly Gibbs, a spokeswoman for the Metropolitan Washington Airports Authority, which operates Dulles International and Reagan National Airport, said it's "too early to speculate" on the implications of the increased MARC service on airport traffic, but that service is welcomed.

"Anything that increases access, transportation access, to the region, to our airports, we view as a good thing," Gibbs said.

All told, O'Malley announced $1.5 billion in transportation projects Wednesday, paid for with funding from a gas tax increase passed this year and including Baltimore's Red Line light rail and improvements on roads and highways near BWI.

Wiedefeld said transportation projects that increase connectivity in the region are welcome.

That is especially true, he said, as the airport weathers the federal budget cuts known as the sequester, which have diminished business travel among federal employers and contractors and personal travel among federal employees facing furloughs.

Transportation advocates also cheered the expanded MARC service.

Ragina Averella, a spokeswoman for AAA Mid-Atlantic, said in a statement that the expansion "is a positive step in providing affordable transportation options and a beneficial service for those wishing to travel from Baltimore to Washington on weekends, as well as to the airport."

Michele Whelley, president and CEO of the Central Maryland Transportation Alliance, which fought for the expansion, said it "will benefit every community between Baltimore and Washington, and will support job growth, residential communities, the entertainment industry, access to services, etc."

Sworn in earlier this month, James T. Smith Jr. is Maryland's 13th transportation secretary. At an age when many people have begun or are contemplating retirement, Smith, 71, is taking the helm of a 10,000-employee agency that oversees highways, bridges, tunnels, BWI Marshall Airport, the...